A combination vaccine approach utilizing replication competent adenovirus-HIV or SIV-recombinants to prime immune responses followed by boosting with subunit protein elicits humoral, cellular, and mucosal immune responses in non-human primates. Significant protection against infection and/or disease progression has been demonstrated in chimpanzee and macaque models. These results have provided the basis for moving the strategy forward into human phase I trials. The first trials will be conducted using replication competent Ad4-HIV recombinants, as Ad4 wild type vaccines have an extensive safety and efficacy record in people. To further develop our AIDS vaccine approach we have generated a number of new recombinants including Ad4-based recombinants encoding "codon-optimized" HIV genes for enhanced expression of HIV gene products, and Ad7 recombinants constructed using a new cosmid system. Recombinants based in different Ad serotypes provide a mechanism for sequential immunization that circumvents host immune responses elicited by the immunizing vector itself. The Ad7 cosmid vectors will be useful not only for construction of HIV recombinants but also can be applied more generally as gene therapy vectors. In preclinical vaccine studies we have investigated in greater depth SIV-specific cellular immune responses elicited by the vaccine regimen, using state-of-the-art methods to quantify CD8+ T-cell responses. We have shown that the replication competent Ad-recombinant approach elicits SIV-specific CD8+ T-cell responses in rhesus macaques equivalent to or greater than other vaccine approaches utilizing live recombinant vectors. In vaccine challenge experiments we have shown that the protective efficacy achieved is correlated with the number of IFN-gamma-secreting cells induced in response to SIV peptide epitopes. The rhesus macaque model is critical for preclinical AIDS vaccine studies. Our studies of a rhesus macaque resistant to SIV infection has shown that a host mechanism, leading to a block in the viral replication cycle post-cellular entry is responsible for the resistance observed. This observation should lead to methods of screening the macaque population for innate resistance, thereby improving the animal model by allowing selection of susceptible macaques with a consequent reduction in the viral challenge dose necessary for infecting control subjects. Preclinical challenge experiments may therefore be conducted using doses of challenge virus more comparable to the doses encountered in natural human infection. Further, the finding provides the platform for further studies investigating the underlying biochemical or immunologic mechanism. The outcome will provide insight into human host-resistance mechanisms which may be exploited for therapeutic or prophylactic purposes. AIDS title: Adenovirus-Recombinant/Subunit Protein Vaccines for AIDS.